This invention relates to methods and apparatus for the control of the phase and magnitude of light, and more particularly, to an endless polarization transformer for transforming the state of polarization (SOP) of light.
In a typical optical communication system, an optical transmitter generates an optical beam and modulates the beam with an electrical signal representative of the information to be transmitted by the communication system. An optical fiber propagates the modulated optical beam to a receiver that demodulates the optical beam to recover the electrical signal. Fiber amplifiers, disposed at appropriate intervals in the fiber optic link between the transmitter and the receiver, maintain the strength of the modulated optical beam. The low loss, light weight, small size, flexibility and high intrinsic bandwidth of optical fiber make optical communication systems highly desirable for the communication of both of digital and analog signals. Examples of optical communication systems include cable TV (CATV) systems, telephone and other cross-country or cross-continent communication systems, and other microwave and RF systems, such as phased array antenna systems used by the military. DWDM (Dense Wavelength Division Multiplexing) systems are also becoming increasingly popular, as they increase the bandwidth of existing optical fibers by transmitting multiple beams, each of a different wavelength and representing a different channel for the communication of information, over the optical fiber.
One important concern with optical communication systems is control of the polarization of the light beam (referred to herein as the state of polarization, or SOP) received by the various optical components in the optical communication system. Polarization is property of light relating to the direction in space of the vibrations of the electromagnetic fields of which light is composed. For example, the electric field of the light can vibrate along either or along both of two orthogonal axes of a coordinate system, where the orthogonal axes of the coordinate system define a plane that is perpendicular to a third axis along which the light propagates. For example, assume that a light beam is propagating along the z axis. The electric field can vibrate solely along the direction of the x axis, can vibrate solely along the direction of the y axis, or can vibrate along both axes, in which case the SOP of the light is determined by the superposition of the orthogonal polarization components, i.e., of the vibrations along the y axis and along the x axis. The superposition can result in the electric field vector tracing out particular shapes in the x-y plane, such as a circle or an ellipse. The shape and the direction in which it is traced (counterclockwise or clockwise) are determined by the relative phase and magnitude of the orthogonal polarization components of the light beam, and correpsond to a particular SOP.
Transmitters and receivers, as well as other components often present in an optical communication system, such as modulators and photonic switches, add-drop multiplexors, etc., are typically designed to operate with light having a particular SOP. However, as is well known to those of ordinary skill in the art, the SOP of a light beam often changes as the beam propagates in the optical communication system. These changes can be random over time. Thus although the transmitter may transmit the proper SOP that would result, if unchanged, in efficient operation of the other components in the optical communication system, the SOP changes as the beam propagates, and the performance of one or more of the components, and hence of the overall optical communication system, can suffer. The use of polarization maintaining optical fiber can reduce the problem of changes of the SOP, but such optical fiber is expensive, does not typically totally eliminate changes of the SOP, and hence is typically only used in short lengths to interconnect adjacent components.
Polarization transformers are known in the art. A polarization transformer can receive the light beam prior to delivery of the beam to the receiver or other optical component and transform the SOP of the beam to the SOP that the receiver or other component is designed to process. For example, it is known in the art that any input SOP can be transformed into any desired output SOP by a cascade of two quarter-wave plates and a half-wave plate. The half-wave plate is placed in between the quarter-wave plates, and each plate is disposed at a selected angle, where the angles can be, and often are, selected to be different. Varying the selected angles varies the SOP to which the input polarization of the beam is transformed.
Mechanical polarization transformers that use a cascade of quarter-wave and half-wave plates are available. However, such mechanical devices are inherently slow, bulky, complex, and not readily miniaturized. Polarization transformers are often combined with devices that track the input or output SOP and a controller for automatically controlling the transformer such that the varying input SOP is continuously transformed to the desired output SOP, which usually should not vary over time.
Another device known in the art is a xe2x80x9cfiber squeezerxe2x80x9d polarization transformer. However, this device is also mechanical and can require the use of complicated xe2x80x9cresetxe2x80x9d algorithms. xe2x80x9cResetxe2x80x9d is a known phenomenon wherein a polarization transformer is controlled by a parameter that can only vary over a finite range. As the tracking and controlling described above progresses, the control parameter can approach one of its limits, and must be reset to a value within the control parameter range to continue to properly transform the SOP. During the reset the transformed polarization varies from the desired output SOP, and this can result in the loss of signal, and hence information received by the receiver or other component while the transformer is reset. Such loss is considered unacceptable in many applications. A polarization transformer that does not require a reset is known in the art as an xe2x80x9cendlessxe2x80x9d polarization transformer, and is more desirable. The quarter-wave and half-wave transformer cascade discussed above is one example of an endless polarization transformer, because the waveplates can be endlessly rotated without reaching a limit.
Other optical apparatus useful for endless polarization transformation are disclosed in U.S. Pat. Nos. 5,212,743; 5,359,678; and 5,361,270, issued May 18, 1993, Oct. 25, 1994, and Nov. 1, 1994, respectively, and all of which include Heismann as an inventor. These patents are referred to herein as the xe2x80x9cHeismann patents,xe2x80x9d and all are herein incorporated by reference. A polarization transformer using the apparatus disclosed in the Heismann patents uses cascaded electrode sections and a titanium indiffused optical waveguide realized on z-propagating lithium niobate. The cascaded electrode sections apply electric fields to the optical waveguide for inducing a selected phase shift between, and a selected relative amplitude between, the orthogonal components of the polarization of the light propagated by the optical waveguide. (In such a waveguide, the orthogonal components of polarization of the light are often referred to as the TM mode and the TE mode. Transfer of power between the orthogonal polarization components is referred to as mode conversion, and changing the relative phase of the orthogonal polarization components referred to as mode phase shifting.) The voltages are placed on the electrodes to produce electric fields that vary, via the electrooptic effect of the electrooptic lithium niobate substrate, the above conversion and phase shift such that the overall SOP determined by the superposition of the TE and TM modes is the desired output SOP.
The devices disclosed in the Heismann patents are often constructed and operated as the electronic analogs of the quarter-wave plate, half-wave plate, quarter-wave plate mechanical device described above, without, however, the limitations on the speed of operation. Although the devices are useful, readily miniaturized and represent an advance in the art, rather high voltages can be required on the electrodes to produce the necessary electric fields. Producing and controlling such voltages can increase the complexity and expense of optical communication systems. Furthermore, the loss introduced by the apparatus is different for each of the orthogonal polarization components, which can result in the transformed SOP differing from the desired polarization.
Liquid crystal polarization transformers and guided wave polarization transformers are also known in the art, but can suffer from the one of more of the disadvantages noted above in reference to other types of transformers, such as being too slow for many applications.
Additional information on known polarization transformers can be found in xe2x80x9cPolarization-State Control Schemes for Heterodyne or Homodyne Optical Fiber Communications,xe2x80x9d Takanori Okoshi, Journal of Lightwave Technology, Vol. LT-3, No. 6, December 1985, pages 1232-1237, herein incorporated by reference.
Accordingly, it is an object of the present invention to address one or more of the foregoing deficiencies and disadvantages of the prior art.
It is another object of the invention to provide improved methods and apparatus for the control of the relative phase and magnitude of light beams, such as can be useful in applications such as the transformation of the polarization of light.
Other objects will be apparent to those of ordinary skill in the art in light of the following disclosure, including the claims.
In one aspect, the invention provides an optical apparatus for transforming the polarization of light from an input polarization to a selected output polarization. The optical apparatus includes a pair of electrooptic optical waveguides, each electrooptical waveguide of the pair having an input and an output, and the pair having at least one coupled section. A plurality of electrodes are provided for exposing the optical waveguides of said pair to selected electric fields. The apparatus also includes one or both of an input beam splitter and a beam combiner.
The input beam splitter includes an input and two outputs. The input receives light having the input polarization and splits the light into two outputs. Each of the outputs is in optical communication with a different input of the optical waveguides of the pair and this optical communication is achieved so as to provide substantially copolarized light to the electrooptic optical waveguides of the pair.
The optical combiner includes first and second combiner inputs and a combiner output. Each of the inputs is in optical communication with a different output of the optical waveguides of said the of optical waveguides, and this optical communication is achieved such that the inputs receive light from the pair of optical waveguides as substantially orthogonally polarized.
In operation, selective application of voltages to the electrodes produces the electric fields such that the light having the input polarization and entering either one of the inputs of the electrooptical optical waveguide pair or, if the beam splitter is present, the input of the beam splitter, is transformed to light having the selected polarization and emanating from one the outputs of one of the electrooptic optical waveguides, or when present, the output of the beam combiner.
The optical apparatus can include a time delay element for providing a selected time delay between the light signal emanating from the two optical waveguides. The time delay element interposed between one of the waveguides and one of the inputs to the combiner. The time delay can be added for compensating for polarization mode dispersion. Also, the invention can include a linearizing element disposed upstream of the beam splitter such that the input polarization of the light received by the beam splitter is always a linear polarization. A polarization rotator can be disposed between one of the outputs of the electrooptic optical waveguides of the pair and one of the inputs of the combiner for achieving the optical communication between the combiner and the pair of optical waveguides such that the light received by the inputs of the combiner is substantially orthogonally polarized.
The beam splitter can be a polarizing beam splitter, where the outputs thereof provide substantially orthogonal components of the light entering the input of the beam splitter. A polarization rotator is disposed between one of the outputs of the beam splitter and one of the inputs of the electrooptic optical waveguides of the pair for achieving the optical communication with the optical waveguides such that the light provided to the optical waveguide is substantially copolarized.
The plurality of electrodes can include a plurality of sections of coplanar electrodes, where each section includes an inner electrode spaced from a pair of outer electrodes by a pair of gaps. For each coplanar electrode section a selected length of each optical waveguide of the pair is disposed proximate a different gap of the pair of gaps for being exposed to electric field lines extending between those electrodes spaced so as to form the different gap. The at least one coupled section can include a plurality of coupled sections of the pair of optical waveguides, the coupled sections being alternately interposed with the sections of coplanar electrodes.
The at least one coupling section can include the optical waveguides of the pair being disposed within a selected distance of each other for a selected coupling length along each of the optical waveguides. The at least one coupling section can also include a junction between the optical waveguides of the pair of optical waveguides.
The optical apparatus of the invention can also include a SOP element for providing a feedback signal responsive to the output polarization and a controller responsive to the feedback signal and in electric communication with the electrodes for selectively applying voltage to the electrodes responsive to the feedback signal.
In yet another aspect, the invention provides an optical apparatus for selectively controlling the polarization of light. The apparatus includes at least first and second transformer sections. Each of the transformer sections includes the following: an electrooptic substrate material; a pair of optical waveguides disposed with the electrooptic substrate material; and at least one section of coplanar electrode structure disposed with the electrooptic substrate material, where the coplanar electrode section includes an inner electrode spaced from a pair of outer electrodes by a pair of gaps such that a first length of each optical waveguide of the pair is disposed proximate a different gap of the pair of gaps for being exposed to electric fields developed proximate the different gap. The optical waveguides of the pair are selectively optically coupled for selectively transferring light energy therebetween.
In addition, the pairs of optical waveguides of the transformer sections are in optical communication to form first and second optical waveguides each having an input end and an output end downstream of the input end. The optical apparatus also includes a polarization splitter having an input and two outputs, the outputs for providing substantially orthogonally polarized components of the light entering the input of the polarization splitter, one of the outputs in optical communication with the input of the first optical waveguide and the other of the outputs in optical communication with the input of the second optical waveguide, the optical communication being achieved such that the light received by the first and second optical waveguides is substantially copolarized. Also included is an optical combiner having first and second combiner inputs and a combiner output. One of the combiner inputs in optical communication with the output of the first optical waveguide and the other the combiner inputs in optical communication with the output of the second optical waveguide. Selective application of voltages between the inner electrodes and the outer electrodes of the transformer sections can selectively transform light of a predetermined polarization entering the input of the input polarization splitter into output light, emanating from the output of the optical combiner, having a selected polarization.
In another aspect, there is provided according to the invention an optical apparatus for selective control of the phase and magnitude of light, where the optical apparatus includes an electrooptic substrate and an elongate, substantially planar electrode section disposed with the electrooptic substrate, and first and second optical waveguides disposed with the substrate for propagating first and second optical beams, respectively. The electrode section includes an inner electrode disposed between first and second outer electrodes spaced from the inner electrode to form first and second gaps therewith, respectively. A first length of the first optical waveguide is disposed proximate the first gap so as to be exposed to electric field lines extending between the inner and first electrodes and a length of the second optical waveguide is disposed proximate the second gap so as to be exposed to electric field lines extending between the inner and second outer electrodes. The first optical waveguide has an input and an output, and the second optical waveguide has an input and an output.
The lengths of the first and second optical waveguides are further disposed for providing a selected optical coupling therebetween, whereby voltages of the same polarity applied to the outer electrodes relative to the inner electrode primarily affects one of the phase difference between the first and second beams and energy transfer between the first and second beams and voltages of opposite polarity applied to the first and second electrodes relative to the inner electrode primarily affects the other of the phase difference and the energy transfer.
In yet a further aspect, according to the invention there is provided an optical apparatus for selective control of the phase and magnitude of light. The optical apparatus includes an electrooptic substrate, first and second optical waveguides disposed with the substrate for transmitting first and second optical beams, respectively, where each optical waveguide of the pair has an input and an output downstream of the input end, and a plurality of first and second section types alternately interposed. The first section type includes an elongated coplanar electrode section disposed with the electrooptic substrate, the coplanar electrode section including an inner electrode spaced from a pair of outer electrodes by a pair of gaps wherein a selected length of each optical waveguide of the pair is disposed proximate a different gap of the pair of gaps for being exposed to electric fields extending between those electrodes spaced so as to form the different gap. The second section type includes a selected length of the pair of optical waveguides wherein the optical waveguides are spaced from each other so as to provide a selected optical coupling therebetween along the selected length. The plurality of first and second section types includes at least two sections of the first type and two sections of the second type. In operation, a first selected voltage applied between the inner and outer electrodes of one of the first type sections can vary the relative phase between the first and second beams and a second selected voltage applied between the inner electrode and outer electrodes of another of the first type sections can vary the relative magnitude of the first and second beams.
The invention also includes methods practiced in accordance with the teachings herein.
In one aspect, the invention provides a method of transforming the polarization of light from a predetermined polarization to a selected polarization, where the method includes the following steps: splitting an input beam of the light of the predetermined polarization into first and second beams; providing first and second electrooptic optical waveguides, selected lengths of the optical waveguides being optically coupled; providing the first and second beams, respectively, to the first and second electrooptic optical waveguides, each having a respective output, the step of providing including providing the first and second beams to the optical waveguides as substantially copolarized; selecting first and second electric fields to apply, respectively, to the first and second electrooptic optical waveguides; applying the first and second selected electric fields, respectively, to the first and second electrooptic optical waveguides; and combining the first and second beams downstream of the outputs of the first and second electrooptic optical waveguides to form an output beam having the selected polarization. The step of selecting the first and second electric fields includes selecting the fields such that the beams, when combined, form the output beam having the selected polarization.
In another aspect, the invention provides a method of transforming the polarization of light from a predetermined polarization to a selected polarization, where the method includes the following steps: splitting an input beam of the light of the predetermined polarization into first and second beams having substantially orthogonal polarizations; copolarizing the beams; adjusting at least one of the relative phase and the magnitude of the two copolarized beams; orthogonally polarizing the beams; and combining the two beams to form an output beam having the selected polarization, wherein the step of adjusting includes adjusting the at least one of the relative phase and the magnitude of the beams such that the beams, when orthogonally polarized and combined, form the output beam having the selected polarization.
Note that if the light having the predetermined polarization is known to be substantially linearly polarized, or if the some loss of power can be tolerated, the step of splitting the beam need not always be performed. For example, in yet a further aspect of the invention, there is provided a method of transforming input light having a predetermined polarization to output light having a selected polarization, where the method includes the following steps: providing a pair of optical waveguides including at least one coupled section, each optical waveguide of said pair having an input and an output; providing the input light to at least one of the inputs of the pair of electrooptic optical waveguides; varying the index of refraction of at least one section of at least one of waveguides of the pair; rotating the polarization of the light emanating from the output of one of the optical waveguides such the rotated polarization is orthogonal to the polarization of light emanating from the output of the other electrooptic optical waveguide of the pair; and combining the light having the rotated polarization with the light emanating from the output of the other electrooptic optical waveguide to form output light. The step of varying the index of refraction includes varying the index such that the input light having the predetermined polarization is transformed to the output light having the selected polarization. If desired, the input light can be linearized prior to providing the input light to one or both of the optical waveguides of the pair of optical waveguides. When the step of providing a pair of optical waveguides includes providing a pair of electrooptical waveguides, the input light tends to be linearized as it propagates along the electrooptic optical waveguide, if the waveguide propagates predominantly the one of the TM or TE modes, and not the other of the TM and TE modes.
If it is desired to provide output light having a substantially linear polarization, it is not always necessary to perform the step of combining the light from the outputs of the optical waveguides. For example, according to yet an additional aspect of the invention, there is provided a method of transforming input light having a predetermined polarization to output light having a selected polarization, where the method includes the following steps: splitting the input light into two orthogonally polarized beams; rotating the polarization of the light of one of the beams such that the light of the beams is copolarized; providing a pair of optical waveguides including at least one coupled section, each optical waveguide of said pair having an input and an output; providing each of the beams to different input of said optical waveguides of the pair of optical waveguides; and varying the index of refraction of at least one section of at least one of waveguides of the pair such that the light meaning from one of the outputs of the electrooptical waveguides of the pair has the selected polarization.
The steps of providing the pair of optical waveguides recited above can include providing a pair of electrooptic optical waveguides, and the steps of varying the index of refraction of at least one section of at least one of the waveguides can include applying a selected electric field to one or both of the optical waveguides for varying the index of refraction via the electrooptic effect.